Locomotives renumbered to 400 through 430 were initially set up for use in branchline or secondary line service to replace steam locomotives in those assignments, mostly in California, Texas and Oklahoma. The 400's varied from single cab units to complete A-B-B-A sets. The single unit and two unit 400's were equipped with footboards and rear headlights, which they retained until deliveries of new GP7s allowed their return to mainline freight service.

Later yet, the railroad renumbered many units to again create A-B-B-A sets, but most units involved in this renumbering did not get their original numbers back. Most of this renumbering came out of the 400 series, but some units did retain their 400 series numbers to the end of their careers.

Some units were renumbered as many as six times over the course of the years. Total FT numbers produced for the Santa Fe was 155 cabs and 165 boosters.

Santa Fe's FTB's were equipped with hostler controls that would allow limited operation of the booster unit. These rudimentary controls were located at the left rear of the engine resulting in an extra porthole on the left side of all Santa Fe FTB's. This fifth porthole was mounted on hinges and could be opened by the hostler so he could see where he was going. The hostler could also operate a small air horn which was located on the upper end wall of the carbody nearest the hostler control station. All Santa Fe's F3, F7 and F9 booster units had hostler controls, hinged porthole and small air horns.